This invention generally relates to systems and methods for repairing structures comprised of composite materials, and in particular, to methods and systems for effecting such repairs with limited resources and time. In particular, the invention relates to systems and methods that use double vacuum debulking to repair structural components made of composite material, such as carbon fiber/epoxy composite material.
The use of structures comprised of composite materials has grown in popularity, particularly in such applications as aircraft airframes, where benefits include increased strength and rigidity, reduced weight and reduced parts count. When damaged, however, composite structures often require extensive repair work which may ground an aircraft, thereby adding significantly to the support costs of the aircraft. Current maintenance procedures frequently require that the damaged component be removed and replaced before the aircraft can resume flying.
Commercial airlines today do not have the ability to repair structural damage to an aircraft's composite structures without severely delaying or canceling the aircraft's next flight. Short domestic flights may have only 30-60 minutes of time at the gate whereas longer and international flights may have 60-90 minutes. The Commercial Airline Composite Repair Committee, an international consortium of airlines, OEMs, and suppliers, reports, however, that the average composite repair permitted in the Structural Repair Manuals takes approximately 15 hours to complete. In most cases, flight cancellations occur when a composite repair is performed at the flight line. Removing an airplane from revenue service in order to repair a damaged composite structure costs the operator the labor to repair the structure and adjust its flight schedules, and also causes passenger dissatisfaction.
In addition to the long time required, repairs to composite structures require special skills and equipment in order to properly carry out the repair. Currently, line mechanics do not generally have the training and experience to perform composite repairs, requiring airlines to call out their composite repair technicians from the back shops or heavy maintenance organizations to perform the repairs. These technicians are often located some distance from the airport gate.
Moreover, in conjunction with the lack of training by line mechanics in repairing structural damage, there are restrictions on what equipment and tools are permitted or available at the flight line. This limits the scope of repairs that can be done at the gate. Standard procedures for repairing composite structures require special tools, equipment, and consumable materials. This may include tools for cutting, grinding and drilling; non-destructive inspection equipment; rulers, guides, and templates; electrical heating and vacuum sources and controllers; and an array of vacuum bagging materials. In many situations, the standard hot bonding equipment required for bonded composite repairs is not allowed at the gate due to the potentially explosive conditions that may be present there.
It is known to provide a repair method and system that uses double vacuum debulking (DVD) to repair a composite structure. As used herein, the term “double vacuum debulking” means a process in which a stack of repair plies is subjected to vacuum and mild heating without being compacted by atmospheric pressure. The purpose of the double vacuum debulk process is to minimize porosity in the repair plies by drawing air and other gases from between the plies while they are in an uncompacted state.
The current DVD process for repairing graphite/epoxy primary and secondary structures requires extensive preparation and multiple process steps to perform acceptable composite repairs. The steps required for processing successful composite repairs is very time consuming and labor intensive. Also, processing repairs on contoured surfaces requires fabrication of support mold structure, which also adds significant time and cost to the production of acceptable repair patches. For example, one existing solution requires the composite repair patch to be processed in an off-aircraft chamber to perform debulking (i.e., forming, degassing, consolidating, and staging the repair patch) for subsequent application onto the structure being repaired. More specifically, the current DVD composite repair system is an off-aircraft process using support tooling (i.e., a vacuum box) to provide the first stage of producing a degassed, consolidated composite repair for application onto parent structure. The repairs in this process are produced on either flat or contoured support tooling, and consolidated using the first stage of the DVD process. The processed repair patch is then transferred to the parent aircraft structure and is cured in a second stage process using an adhesive layer in the bondline interface. Flat DVD patches can be fabricated and applied to contoured structure, but when the contour becomes complex or radical, then separate contour matched tooling is required to fabricate the DVD repair.
The set-up for the above-described existing solution involves support mold tooling that allows for the DVD processing to be performed as a separate step. The repair patch is heat bonded to the primary (i.e., parent) structure in need of repair and fully cured in a separate step. Significant processing time, material, and tooling cost savings can be realized if these two separate process steps used in the current DVD repair system could be combined into one set-up on the structure to be repaired.
There is a need for a DVD process in which multiple steps are combined into one set-up. Using such process, the repairs can be completed using the parent structure as a replacement for a forming mandrel for the repair patch consolidation, and the repaired structure can be used for support structure tooling for patch co-cure processing.
Furthermore, metallic structures are sometimes repaired using high-modulus fibers, including carbon and boron materials. The process of repairing metallic structures with composite materials would also benefit from an in-situ DVD process that reduced cycle times for set-up and repair processing.